Surface alteration of (VO)2P2O7 by α-Sb2O4 as a route to control the n-butane selective oxidation Original Research Article

The catalytic properties of (VO)2P2O7/α-Sb2O4 mixed oxides system for n-butane mild oxidation have been investigated on two mechanical mixtures (M1 and M2) of the same well crystallized (VO)2P2O7 (reference vanadyl pyrophosphate) with two different morphologies of α-Sb2O4. The M1 mixture of (VO)2P2O7 with α-Sb2O4 (1), prepared by oxidation of Sb2O3, leads to the oxidative dehydrogenation (ODH) of n-butane, whereas the M2 mixture of (VO)2P2O7 with a commercial α-Sb2O4 (2) (Aldrich) with a different morphology improves the maleic anhydride selectivity as compared to the reference (VO)2P2O7 catalyst (synergetic effect). After reaction, no ternary VPSbO phase is detected by XRD and DTA and it was controlled that the two α-Sb2O4 oxides are catalytically inactive. The (VO)2P2O7 reference catalyst which produced only maleic anhydride as mild oxidation product shows by XPS a slightly oxidized surface (14% V5+–86% V4+). Contamination of the (VO)2P2O7 phase by migration of Sb species occurs after catalytic reaction in the case of the M1 mixture as shown by XPS, LEIS and TEM–EDX analysis. XPS showed that (VO)2P2O7 is partially superficially reduced (86% V4+–14% V3+). This feature is consistent with the decrease of acidity as observed by pyridine adsorption–desorption. In opposition with the M1 mixture, no contamination of the (VO)2P2O7 phase is observed after catalytic reaction in the case of the M2 mixture. The XPS study shows, in this case, that (VO)2P2O7 is partially oxidized (30% V5+–70% V4+) at a higher level than for the reference (VO)2P2O7 catalyst. This situation is associated with the increase of selectivity observed for maleic anhydride (synergetic effect). The difference in the catalytic results for the two M1 and M2 mixtures, as compared to the (VO)2P2O7 reference catalyst, can be explained by the alteration of the surface composition of (VO)2P2O7 and the distribution of vanadium oxidation state due to different interaction between Sb2O4and (VO)2P2O7, depending on the orientation of the α-Sb2O4 crystals.